Is UV mapping and texturing necessary? Can I just use materials before rigging?
You don't have to texture a character before you rig it and you don't have to UV unwrap it if a procedural texture works.

While a UV wrapped texture can be used to control the glossy factor in precise parts of the character, you can turn down the overall glossiness of a procedural texture, the steps will depend on which render engine you use and the material you have.

I think it is better to rig a character before unwrapping (especially when learning) as the rigging process can lead to finding bad mesh deforms that need better topology to fix, this leads to an existing UV unwrap needing to be adjusted for the changes.